Implement Supply.throttle for Callables
Implement Supply.throttle for Callables

If the second positional parameter to Supply.throttle is a Callable,
it is assumed to be a block to be executed asynchronously restrained
to the limit given.  This basically reduces the asynchronous image
resizer example in http://jnthn.net/papers/2014-yapceu-async.pdf to:

  @worklist.Supply.throttle(PARALLEL_RESIZE,&resizer-one).wait

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